1. Field of the Invention
The present invention relates to methods for disposing underfill and adhesive materials during microelectronic package fabrication. In particular, the present invention relates to utilizing radiation curable materials, which, when exposed to such radiation, prevents excessive bleed-out during the fabrication of the microelectronic packages.
2. State of the Art
In the field of electronic systems, there is continuous competitive pressure to increase the performance of components while reducing production costs. This competitive pressure is particularly intense in the fabrication of microelectronic devices, where each new generation must provide increased performance while also reducing the size or footprint of the microelectronic device.
As shown in FIG. 17, an exemplary microelectronic package includes a microelectronic die 202 that is mounted on a substrate 204, such as an interposer, a motherboard, and the like, which functionally connects the microelectronic die 202 through a hierarchy of electrically conductive paths (not shown) to the other electronic components (not shown). The illustrated method for electronically mounting the microelectronic die 202 to the substrate 204 is called flip chip bonding. In this mounting method, electrically conductive terminals or pads 206 on an active surface 208 of the microelectronic die 202 are attached directly to corresponding lands 212 on a surface 214 of the substrate 204 using reflowable solder bumps or balls 216, thermocompression bonding, or any other known methods of flip chip attachment.
To enhance the reliability of the solder bumps 216 connecting the microelectronic die pads 206 and the substrate lands 212, an underfill material is used to mechanically and physically reinforce them. In a known method of underfill encapsulation shown in FIGS. 18 and 19, a low viscosity underfill material 222, such as an epoxy material, is dispensed from at least one dispensing needle 230 along at least one edge 224 (usually one or two edges) of the microelectronic die 202. The underfill material 222 is drawn between the microelectronic die 202 and the substrate 204 by capillary action (in generally the x-direction shown as arrows 240 in FIG. 19), and the underfill material 222 is subsequently cured (hardened) using heat, which forms the microelectronic package 200 shown in FIG. 20.
With the pressure to decrease the size of the microelectronic packages, bump pitch 226 and bump height 228 has decreased. Thus, it has become successively more difficult to obtain adequate underfill material dispersion without continuously decreasing the viscosity of the underfill material 222 or improving its wettability properties. However, decreasing the viscosity and/or improving the wettability of the underfill material 222 results in the underfill material 222 bleeding out and substantially surrounding the microelectronic die 202, as shown in FIGS. 20 and 21. This bleed-out area beyond the edges 224 of the microelectronic die 202 is generally referred to as a “bleed-out tongue” 232 having a varying width 234. The bleed-out tongue 232 is a problem because it can cover and contaminate valuable surface area on the substrate 204.
For example, as shown in FIG. 22, an exemplary stacked package 250 includes a microelectronic die 202 that is mounted on a substrate 204 with a plurality of solder bumps 216 extending between microelectronic die pads 206 and substrate lands 212, as discussed with regard to FIG. 17. A second microelectronic die 242 is attached by its back surface 244 to a back surface 246 of the microelectronic die 202 with a layer of adhesive 248. A plurality of wirebonds 252 makes electrical contact between lands 254 on an active surface 256 of the second microelectronic die 242 and wirebond lands 258 on the substrate 204. The substrate wirebond lands 258 are placed as close to the microelectronic die 202 as possible in order to conserve the valuable surface area in the substrate 204 and also meet chip scale package small form factor requirements. However, FIG. 22 illustrates the stacked package 250 without an underfill material. As shown in FIG. 23, the underfill material 222 is disposed before the wirebonds 252 (see FIG. 22) are attached. However, the bleed-out tongue 232 can have a width 234, which covers the wirebond lands 258. Thus, at least the portion of the bleed-out tongue 232 covering the wirebond lands 258 would have to be removed in order to attach the wirebonds 252 (see FIG. 22). This, of course, is difficult and may reduce the reliability of the microelectronic device, as well as increasing the package cost.
As shown in FIG. 24, the bleed-out problem also exists when the second microelectronic die 242 of FIG. 23 is directly attached by its back surface 244 to the substrate 204 rather than being stacked. An adhesive material 262 is disposed between the substrate 204 and the second microelectronic die 242. When the second microelectronic die 242 is positioned and placed on the substrate 204, the adhesive material 262 can bleed-out and cover the wirebond lands 258, which would have to be removed in order to attach the wirebonds 252 (see FIG. 22). This, again, is difficult and may reduce the reliability of the microelectronic device, as well as increasing the package cost.
Therefore, it would be advantageous to develop techniques to effectively dispose underfill and adhesive materials between a microelectronic die and the substrate while substantially reducing the bleed-out.